U.S. patent number 8,163,647 [Application Number 12/458,308] was granted by the patent office on 2012-04-24 for method for growing carbon nanotubes, and electronic device having structure of ohmic connection to carbon element cylindrical structure body and production method thereof.
This patent grant is currently assigned to Fujitsu Limited. Invention is credited to Akio Kawabata, Mizuhisa Nihei.
United States Patent |
8,163,647 |
Kawabata , et al. |
April 24, 2012 |
Method for growing carbon nanotubes, and electronic device having
structure of ohmic connection to carbon element cylindrical
structure body and production method thereof
Abstract
An electronic device having a structure of an ohmic connection
to a carbon element cylindrical structure body, wherein a metal
material is positioned inside the junction part of a carbon element
cylindrical structure body joined to a connection objective and the
carbon element cylindrical structure body and the connection
objective are connected by an ohmic contact. Methods for producing
such an electronic device are also disclosed. Further, a method for
growing a carbon nanotube is disclosed.
Inventors: |
Kawabata; Akio (Kawasaki,
JP), Nihei; Mizuhisa (Kawasaki, JP) |
Assignee: |
Fujitsu Limited (Kawasaki,
JP)
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Family
ID: |
32993007 |
Appl.
No.: |
12/458,308 |
Filed: |
July 8, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090269921 A1 |
Oct 29, 2009 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10773311 |
Feb 9, 2004 |
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Foreign Application Priority Data
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Mar 20, 2003 [JP] |
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2003-078353 |
Mar 25, 2003 [JP] |
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2003-083192 |
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Current U.S.
Class: |
438/652;
438/778 |
Current CPC
Class: |
C23C
16/26 (20130101); B82Y 10/00 (20130101); H01L
21/76838 (20130101); H01L 21/28562 (20130101); B82Y
30/00 (20130101); H01L 21/76879 (20130101); H05K
3/4076 (20130101); H01L 2221/1094 (20130101) |
Current International
Class: |
B32B
9/00 (20060101); B32B 15/04 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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09031757 |
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Feb 1997 |
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JP |
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10203810 |
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Aug 1998 |
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JP |
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11116218 |
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Apr 1999 |
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JP |
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11139815 |
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May 1999 |
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JP |
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2000353467 |
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Dec 2000 |
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JP |
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2001020072 |
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Jan 2001 |
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JP |
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2001303250 |
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Oct 2001 |
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JP |
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2001-358083 |
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Dec 2001 |
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JP |
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2002-110567 |
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Apr 2002 |
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JP |
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2002179418 |
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Jun 2002 |
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JP |
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2002518280 |
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Jun 2002 |
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JP |
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2002-212729 |
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Jul 2002 |
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JP |
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2002530805 |
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Sep 2002 |
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JP |
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2002293524 |
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Oct 2002 |
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JP |
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2004-238258 |
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Aug 2004 |
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JP |
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WO 99/65821 |
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Dec 1999 |
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WO |
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WO 00/30141 |
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May 2000 |
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WO |
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Other References
Choi et al; "Variations in structure and emission characteristics
of nanostructured carbon films prepared by the hot-filament
chemical-vapor-deposition method due to the addition of ammonia in
the source;" J. Vac. Sci. Technol. B 21(1) (Jan./Feb. 2003) pp.
576-580. cited by other .
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Appl. Phys. vol. 41 (2002) pp. L67-L69. cited by other .
Lee et al; "Effects of metal buffer layers on the hot filament
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(2001) pp. 11424-11431. cited by other .
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Tang et al; "Carbon monoxide-assisted growth of carbon nanotubes";
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(2001); pp. 259-264. cited by other .
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and Carbide Nanorods"; Science vol. 285 (Sep. 10, 1999); pp.
1719-1722. cited by other.
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Primary Examiner: Smith; Zandra
Assistant Examiner: Perkins; Pamela E
Attorney, Agent or Firm: Kratz, Quintos & Hanson,
LLP
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a Divisional Application of prior application
Ser. No. 10/773,311, filed on Feb. 9, 2004, now abandoned which is
being hereby incorporated by reference.
Claims
The invention claimed is:
1. A method for producing an electronic device having a structure
of ohmic connection to a carbon element cylindrical structure body,
comprising disposing a metal material on a connection objective
capable of ohmically contacting a carbon element cylindrical
structure body and forming a carbon element cylindrical structure
body by chemical vapor deposition using said metal material as the
catalyst while accomplishing an ohmic contact between the carbon
element cylindrical structure body and the connection objective,
wherein the material of said connection objective is Ti, Nb, Si or
C, and is alloyed with said metal material by the elevation of
temperature during said chemical vapor deposition and a carbon
element cylindrical structure body is grown using the particle of
said metal material in said alloy as the catalyst for said chemical
vapor deposition, and, at the same time, a part of the material of
the connection objective is carbidized to join the carbon element
cylindrical structural body to the connection objective by ohmic
contact.
2. The method for producing an electronic device as claimed in
claim 1, wherein said metal material is Ni, Fe or Co, or an alloy
containing at least one of Ni, Fe and Co.
3. The method for producing an electronic device as claimed in
claim 1, wherein said chemical vapor deposition is performed by
applying an electric field in the growth direction of the carbon
element cylindrical structure body.
4. The method for producing an electronic device as claimed in
claim 1, wherein said carbon element cylindrical structure body is
a carbon nanotube.
5. A method for producing an electronic device having a structure
of ohmic connection to a carbon element cylindrical structure body,
comprising forming a first stack of a first material capable of
ohmically contacting a carbon element cylindrical structure body
and a second material of catalyst metal disposed on said first
material, heat-treating said first stack in vacuum or in a hydrogen
atmosphere to form a second stack made of a lower layer composed of
an alloy of the first material and the second material, an
intermediate layer composed of the first material and an upper
layer composed of a fine particle of the second material, and
forming a carbon element cylindrical structure body by chemical
vapor deposition using the fine particle of the second material on
the surface of said second stack as the catalyst to incorporate the
fine particle of the second material into the inside of the carbon
element cylindrical structure body and at the same time, a part of
the first material in the intermediate layer contacting with the
bottom of the side wall of the carbon element cylindrical structure
is carbidized to join the carbon element cylindrical structure and
the Ti intermediate layer by ohmic contact.
6. The method for producing an electronic device as claimed in
claim 5, wherein said first material is Ti, Nb, Si or C.
7. The method for producing an electronic device as claimed in
claim 5, wherein said second material is Ni, Fe or Co, or an alloy
containing at least one of Ni, Fe and Co.
8. The method for producing an electronic device as claimed in
claim 5, wherein said chemical vapor deposition is performed by
applying an electric field in the growth direction of the carbon
element cylindrical structure body.
9. The method for producing an electronic device as claimed in
claim 5, wherein said carbon element cylindrical body is a carbon
nanotube.
Description
This application is based upon and claims the benefit of priority
from each of prior Japanese Patent Applications No. 2003-078353,
filed on Mar. 20, 2003, and No. 2003-083192, filed on Mar. 25,
2003, the entire contents thereof being incorporated herein by
reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic device having a
structure for ohmic connection to a carbon element cylindrical
structure body represented by a carbon nanotube and a production
method thereof, which techniques are widely applicable when a
carbon element cylindrical structure body is applied to an
electronic device.
The present invention also relates to a method for growing carbon
nanotubes.
2. Description of the Related Art
In recent years, studies are being made on the use of a carbon
element cylindrical structure body as an electrically conducting or
semiconductor material in an electronic device. In the electronic
device, a carbon element cylindrical structure body must be
ohmically connected to an electrode or a wire so as to avoid an
increase in electric resistance in the connection part.
FIG. 1 schematically shows the connection of a carbon element
cylindrical structure body 11, which is formed as a longitudinal
via material in a via hole of an electronic device, to a lower
wiring Cu layer 13 and an upper wiring Cu layer 15. In a via hole
formed in an interlayer insulating film 17 on the lower wiring
layer 13, a bundle of carbon element cylindrical structure bodies
11 is formed in vertical orientation. The carbon element
cylindrical structure body 11 is grown by using a chemical vapor
deposition (CVD) method and, at this time, a catalyst metal (for
example, Ni) layer 19, necessary for the growth of the carbon
element cylindrical structure body, is present on the wiring layer
exposed in the via hole. On the other hand, between the carbon
element cylindrical structure body 11 and the upper wiring layer
15, a Ti layer 21 is inserted.
The connection between the carbon element cylindrical structure
body 11 and the upper wiring layer 15 is an ohmic connection
resulting from carbidization (formation into TiC) of the Ti layer
21 intervening therebetween. This method is a technique of
contacting the carbon element cylindrical structure body with the
Ti layer and then performing a heat treatment at a high temperature
to cause TiC formation at the interface, thereby obtaining an ohmic
connection (see, Y. Zhang et al., Science 285, 1719 (1999)).
As shown in FIG. 1, in the conventional structure, only a catalyst
metal (Ni) layer 19, necessary for the growth of the carbon element
cylindrical structure body 11, is present on the lower wiring (Cu)
layer 13.
With respect to the production method of a carbon nanotube, arc
discharge, laser evaporation, thermal CVD, plasma enhanced CVD and
the like are known. By the arc discharge or laser evaporation
method, a carbon nanotube having good quality can be obtained but
the orientation or length of carbon nanotubes can hardly be
controlled.
The method of enabling the control of the orientation or direction
includes a thermal CVD method and a plasma enhanced CVD method. In
these methods, carbon nanotubes can be orientation-grown by
applying an electric field during the growth. The orientation
growth in the case of not applying an electric field is described
in Nature, Vol. 416, pp. 495-496 (2002), however, the growth
temperature is as high as 800.degree. C. or more and therefore, it
is impossible to grow carbon nanotubes on a semiconductor circuit
by this technique. Furthermore, the growth of carbon nanotubes at
550.degree. C. is reported in Chemical Physics Letters, 360, pp.
2229-234 (2002), however, the growth direction cannot be
controlled.
Various methods for producing carbon nanotubes by using thermal CVD
are described in patent documents. For example, Japanese Unexamined
Patent Publication (Kokai) No. 9-31757 (JP 9-31757 A) discloses a
method of producing graphite nanotubes at a low temperature by CVD,
where the graphite nanotube is produced at 650 to 800.degree. C.
Japanese Unexamined Patent Publication (Kokai) No. 10-203810 (JP
10-203810 A) describes a technique of growing carbon nanotubes on a
substrate at a relatively low temperature, where the growth
requires a plasma produced by a direct-current glow discharge.
Japanese Unexamined Patent Publication (Kokai) No. 11-139815 (JP
11-139815 A) describes a method for producing a carbon nanotube
device by using a thermal decomposition reaction of the starting
material gas. Also, Japanese Unexamined Patent Publication (Kokai)
No. 2001-303250 (JP 2001-303250 A) describes a method of vertically
orienting carbon nanotubes on a substrate by using thermal CVD,
where a direct current voltage is applied during the growth.
In these methods for producing carbon nanotubes by using thermal
CVD, when auxiliary means such as an electric field is not used, a
growth temperature of 500.degree. C. or more is generally used.
A CVD method using a hot filament (hot-filament CVD) is also known.
Japanese Unexamined Patent Publication (Kokai) No. 2000-353467 (JP
2000-353467 A) describes a method for producing a cold cathode
device, where a diamond or diamond-like carbon electron-releasing
material is formed by hot-filament CVD. The production of carbon
nanotubes is not referred to therein. Japanese National Publication
(Kohyo) No. 2002-518280 (JP 2002-518280 A) describes a method for
growing carbon nanotubes by hot-filament CVD. In this method, an
electric field is applied during the growth.
In this way, in conventional production of carbon nanotubes by
hot-filament CVD, application of an electric field is performed as
auxiliary means.
Also, a technique of growing carbon nanotubes at a filament
temperature of 1,600.degree. C. by hot-filament CVD is described in
Chemical Physics Letters, 342, pp. 259-264 (2001).
SUMMARY OF THE INVENTION
The electronic device having a structure of an ohmic connection to
a carbon element cylindrical structure body according to the
present invention is characterized in that a metal material is
positioned inside the junction part of a carbon element cylindrical
structure body joined to a connection objective and the carbon
element cylindrical structure body and the connection objective are
connected by an ohmic contact. By virtue of connection through an
ohmic contact, the increase in resistance at the connected part
between the carbon element cylindrical structure body and the
connection objective can be suppressed.
The electronic device having a structure of an ohmic connection to
a carbon element cylindrical structure body of the present
invention can be produced by a method comprising disposing a metal
material on a connection objective capable of ohmically contacting
with a carbon element cylindrical structure body, and forming a
carbon element cylindrical structure body according to chemical
vapor deposition using the metal material as the catalyst while
accomplishing an ohmic contact between the carbon element
cylindrical structure body and the connection objective. By using
for the connection objective a material capable of ohmically
contacting with a carbon element cylindrical structure body, an
ohmic connection structure therebetween can be realized
simultaneously with the growth of the carbon element cylindrical
structure body.
Alternatively, the electronic device having a structure of an ohmic
connection to a carbon element cylindrical structure body of the
present invention can be produced by a method comprising forming a
first stack of a first material capable of ohmically contacting
with a carbon element cylindrical structure body and a second
material of catalyst metal disposed on the first material,
heat-treating the first stack in vacuum or in a hydrogen atmosphere
to form a second stack made of a lower layer composed of an alloy
of the first material and the second material, an intermediate
layer composed of the first material and an upper layer composed of
a fine particle formed of the second material, and forming a carbon
element cylindrical structure body by chemical vapor deposition
using the fine particle of the second material on the surface of
the second stack as the catalyst to incorporate the fine particle
of the second material into the inside of the carbon element
cylindrical structure body and at the same time, connect, by an
ohmic contact, the side wall of the carbon element cylindrical
structure body to the intermediate layer composed of the first
material. The carbon element cylindrical structure body grows on
the first material capable of an ohmic contact by the action of the
fine particle catalyst, so that the carbon element cylindrical
structure body can be connected by an ohmic contact to the
intermediate layer composed of the first material simultaneously
with the growth of the carbon element cylindrical structure
body.
The term "carbon element cylindrical structure body" as used herein
is a linear nanostructure constituted by carbon atoms and this is a
generic term for a carbon nanotube, a cup-stacked type structure, a
carbon fiber or the like.
The method for growing a carbon nanotube of the present invention
is a method comprising disposing a substrate in a growth chamber,
supplying a starting material gas to the chamber and
orientation-growing a carbon nanotube on the substrate by CVD, the
method being characterized in that neither an electric field nor a
plasma is used for the growth of the carbon nanotube and that heat
generated from a filament disposed in the growth chamber is
utilized.
By using the hot filament, carbon nanotubes can be
orientation-grown at a relatively low temperature, specifically,
even at a substrate growth face temperature of less than
500.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and advantages of the invention will be
well understood and appreciated by a person with ordinary skill in
the art, from consideration of the following detailed description
made by referring to the attached drawings, wherein:
FIG. 1 is a schematic view for explaining a conventional technique
of using a carbon element cylindrical structure body for the
longitudinal via material of an electronic device;
FIG. 2A is a schematic view for explaining a first embodiment of
the present invention, where a carbon element cylindrical structure
body is applied to the longitudinal via of an electronic
device;
FIG. 2B is an enlarged view of the portion denoted by B in FIG.
2A;
FIG. 3A is a view for explaining a laminate film used in the method
for producing a structure of an ohmic connection of the present
invention;
FIG. 3B is a view for explaining a NiTi alloy layer obtained from
the Ni/Ti laminate film of FIG. 3A;
FIG. 4A is a schematic partial enlarged view for explaining the
first stack used in a second embodiment of the present
invention;
FIG. 4B is a schematic partial enlarged view for explaining the
second stack used in the second embodiment of the present
invention;
FIGS. 5A to 5C are schematic views showing the production process
in Example 1;
FIGS. 6A and 6B are schematic views showing the production process
in Example 2;
FIG. 7 is a schematic view for explaining the method for growing
carbon nanotubes of the present invention;
FIGS. 8A and 8B are views for explaining the growth of carbon
nanotubes in Example 3;
FIGS. 9A and 9B are views for explaining the growth of carbon
nanotubes in Example 4; and
FIGS. 10A and 10B are views for explaining the growth of carbon
nanotubes in Example 5.
DETAILED DESCRIPTION OF THE INVENTION
As described above, the term "carbon element cylindrical structure
body" as used herein is a linear nanostructure constituted by
carbon atoms and this is a generic term for a carbon nanotube, a
cup-stacked type structure, a carbon fiber or the like. In the
following, the present invention is described by referring to
carbon nanotube which is one representative of these
nanostructures.
In a conventional structure, such as that shown in FIG. 1, a Ti
layer is not present between the carbon element cylindrical
structure body 11 and the lower wiring layer 13 and therefore, an
ohmic contact by the TiC formation cannot be formed. Furthermore,
as for the contact with the lower wiring layer 13, an ohmic contact
cannot be formed as in the case of the upper wiring layer 15 by
depositing a Ti layer after the growth of the carbon element
cylindrical structure body 11 and, subsequently, performing a
high-temperature heat treatment. That is, in the conventional
structure, an ohmic contact ensuring sufficiently low resistance
cannot be formed between the lower wiring layer and the carbon
element cylindrical structure body and, thus, the wiring/via part
formed by the carbon element cylindrical structure body
disadvantageously has high resistance.
One of objects of a first aspect of the present invention is to
provide an electronic device having a structure of an ohmic
connection to a carbon element cylindrical structure body, which
can be realized simultaneously with the growth of the carbon
element cylindrical structure body, and a production method
thereof.
FIGS. 2A and 2B schematically show an embodiment in which the
present invention is applied to an electronic device having a
longitudinal via. Within a via hole formed in an interlayer
insulating film 37 on a lower wiring (Cu) layer 33, a bundle of
carbon nanotubes 31 is formed in vertical orientation. In the
present invention, as shown in FIG. 3A, a laminate film obtained by
previously forming, on the lower wiring layer 33 in the via hole, a
Ti layer 45 for the TiC formation of the junction part of the
carbon nanotube 31 to the lower wiring layer 33 and forming thereon
a catalyst metal (for example, Ni) layer necessary for the growth
of nanotubes is used. When this Ni/Ti laminate film is used, the
Ni/Ti laminate film is subject to an elevation of the substrate
temperature at the time of growing carbon nanotubes by chemical
vapor deposition and changes into a NiTi alloy layer 45a (FIG. 3B).
The carbon nanotube 31 grows on the surface of the NiTi alloy layer
45a by using, as a nucleus, the catalyst metal Ni fine particle in
the alloy. As shown in FIG. 2B, which is an enlarged view of the
portion denoted by B in FIG. 2A, the Ni fine particle 39 is
embraced in the inside at the root of the growing carbon nanotube
31 and the side wall of the nanotube 31 comes into contact with Ti
remaining on the surface of the alloy layer 45a. This contact
portion 47 becomes the ohmic contact site where Ti is carbidized.
In other words, an ohmic contact can be formed between the alloy
layer 45a and the nanotube 31 simultaneously with the growth of
carbon nanotube.
In fact, when the contact resistance between the nanotube 11 or 31
and the lower wiring layer 13 or 33 was measured for the structures
of conventional example (FIG. 1) and the present invention (FIG.
2), the resistance of the present invention was lower by single or
double figures (conventional example: 15 M.OMEGA., present
invention: 130 k.OMEGA.). This result infers that when the present
invention is used, TiC formation occurs at the contact portion
between the carbon nanotube and the lower wiring layer
simultaneously with the growth of carbon nanotube.
The carbon nanotube 31 formed within the via hole and the upper
wiring layer 35 can be ohmically connected, as in the conventional
art, by the carbidization (TiC formation) of the Ti layer 41
intervening therebetween. In this way, according to the present
invention, the carbon nanotube can be connected by a good ohmic
contact with both the lower and upper wiring layers 33 and 35 and a
via with low resistance can be realized.
In the case described above (first embodiment), the connection
objective to which the carbon nanotube 31 is ohmically connected is
an alloy layer 45a and the metal material thereon is a catalyst
metal fine particle 39.
A second embodiment where the present invention is applied to an
electronic device having a longitudinal via is described below.
Similarly to the first embodiment described by referring to FIGS.
2A and 2B, as shown in FIG. 4A, a first stack 54 made of a Ti layer
52 for the TiC formation of the junction part of the carbon
nanotube to the lower wiring layer 33 and a catalyst metal Ni layer
53 provided thereon is formed on the lower wiring layer 33 within a
via hole. This stack is heat-treated in a vacuum or in a hydrogen
atmosphere to form a second stack 61 constituted by, as shown in
FIG. 4B which is a partial enlarged view similarly to FIG. 2B, a
lower layer 55 of TiNi alloy, an intermediate layer 57 of Ti and an
upper layer of Ni fine particle 59. Subsequently, chemical vapor
deposition is performed by using the Ni fine particle on the
surface of the stack 61 as the catalyst, whereby the Ni fine
particle is incorporated inside the carbon nanotube 51 and, at the
same time, a part of Ti in the intermediate layer 57 contacting
with the bottom of the side wall of carbon nanotube 51 is
carbidized to join the carbon nanotube 51 and the Ti intermediate
layer 57 by ohmic contact.
In this embodiment, the Ti layer 57 corresponds to the connection
objective to which the carbon nanotube is ohmically connected in
the first embodiment, and the Ni fine particle 59 similarly
corresponds to the metal material as the catalyst fine particle in
the previous case.
The connection objective to which the carbon element cylindrical
structure body is connected by an ohmic contact is the NiTi alloy
layer 45a in the first embodiment and this layer is derived from
the Ti layer 45 which is previously formed. In the second
embodiment, the connection objective is the Ti intermediate layer
57 and this layer is also derived from the Ti layer which is
previously formed (Ti layer before heat treatment). In this
meaning, in either embodiment, the connection objective to which
the carbon nanotube is connected can be regarded as a Ti layer for
carbidization (Ti layer before the heat treatment accompanying the
growth of nanotubes (in the first embodiment) or Ti layer formed by
the heat treatment before the growth of nanotube (in the second
embodiment)), that is, a material of undergoing ohmic connection to
the carbon nanotube by carbidization.
In the present invention, Nb, Si or C other than Ti can be used as
the material (connection objective) undergoing an ohmic connection
to carbon nanotube by carbidization. When the material as the
connection objective is C (carbon), the carbon nanotube and the
connection objective are connected by a carbon-carbon bond and, in
the present invention, this connection by a carbon-carbon bond is
also called herein connection by carbidization.
As the metal material acting as the catalyst for the growth of
carbon nanotubes by CVD, Fe or Co can be used other than Ni. An
alloy containing at least one metal selected from Ni, Fe and Co can
also be used.
One of objects of a second aspect of the present invention is to
provide a novel method capable of producing carbon nanotubes
orientation-grown at a low temperature without relying on auxiliary
means such as electric field (or plasma) which has been heretofore
employed and, thereby, to enable the growth of carbon nanotubes on,
for example, a semiconductor circuit which cannot be subjected to a
high-temperature treatment.
In the method for growing carbon nanotubes of the present
invention, a filament disposed in the growth chamber is used as a
heat source for the growth of carbon nanotubes by CVD. Such a CVD
process using a filament for heating (called a hot filament) is
known as hot-filament CVD but in conventional techniques of using
this method for the growth of carbon nanotubes, a filament
temperature of 1,600.degree. C. is required (see, Chemical Physics
Letters, 342, pp. 259-264 (2001)).
The hot filament generates a heat upon passing of a current. The
filament temperature at the growth of carbon nanotubes is
preferably 400.degree. C. or more. If the filament temperature is
less than 400.degree. C., this is insufficient to supply an energy
for decomposing the starting material gas, whereas if it is
unnecessarily elevated, the energy is wasted. Therefore, the
filament temperature in general is preferably from 400 to
1,000.degree. C., more preferably from 400 to 600.degree. C., still
more preferably from 400 to 500.degree. C.
The hot-filament should be produced from a material capable of
enduring high temperature (400.degree. C. or more) at the growth of
carbon nanotubes and at the same time, undergoing no or little
chemical reaction with the starting material gas or a decomposition
product thereof. The present inventors have found that for the
purpose of producing carbon nanotubes by CVD from a starting
material gas containing a carbon source, a filament made of rhenium
or a material mainly comprising rhenium is suitable.
As shown in FIG. 7, a hot-filament 212 is disposed to face a growth
substrate 214 in a vacuum chamber (reaction chamber) 210 to which a
starting material gas is supplied. The distance between the
filament 212 and the substrate 214 is determined according to the
carbon nanotube growth conditions (e.g., the kind of starting
material gas used and the growth rate).
At the growth, the hot filament 212 can be moved above the
substrate 214 to uniformly grow carbon nanotubes in an arbitrary
area. Also, the substrate 214 may be moved by fixing the position
of the hot filament 212. Alternatively, the hot filament 212 and
the substrate 214 both may be relatively moved. As for the mode of
movement, for example, rotation or reciprocation can be employed.
For example, in FIG. 7, the filament 212 can be made to cause a
reciprocating movement in the horizontal direction while moving the
substrate 214 in the vertical direction by the movement of a
substrate stage 216. The filament 212 is connected to, for example,
an A.C. power source 218 and the substrate stage 216 is generally
equipped with substrate heating means (not shown).
A carbon source gas containing carbon is used as the starting
material for the growth of carbon nanotubes. The carbon source gas
may be a hydrocarbon gas such as methane, ethane, acetylene,
propane or butane, or a gas of alcohols such as methanol or
ethanol. A mixture of two or more carbon sources may also be
used.
The starting material gas may contain, in addition to the carbon
source, one or both of a reactive gas such as hydrogen and an inert
gas such as helium or argon.
The total pressure of the starting material gas in the growth
chamber may be approximately from 0.1 to 100 kPa. If the pressure
is less than 0.1 kPa, the growth rate of carbon nanotubes
decreases, whereas if it exceeds 100 kPa, there may be a danger of
the starting material gas leaking out of the growth chamber. The
total pressure of the starting material gas is preferably from 0.1
to 10 kPa, more preferably from 0.3 to 10 kPa.
A substance acting as the catalyst for the growth reaction must be
present on the substrate surface where carbon nanotubes are grown.
As the catalyst, a transition metal such as Fe, Ni, Co or Pd may be
used and an alloy of two or more of these transition metals may
also be used. Furthermore, an alloy of such a transition metal
capable of acting as the catalyst and a metal of not acting as the
catalyst, for example, Fe--Pt and Co--Pt, may also be used.
The catalyst may form a thin film on the surface of the growth
substrate or may be a fine particle dispersed on the substrate
surface. In the case of a fine particle catalyst, the diameter of
the growing carbon nanotube can be controlled by controlling the
diameter of the fine particle. For example, as described in
Examples later, when fine particle catalysts having diameters of
about 7 nm and 4 nm were used, carbon nanotubes having diameters of
about 15 nm and 8 nm could be obtained, respectively.
The thin-film catalyst may be formed by any method. For example,
evaporation or sputtering can be used. The thickness of the thin
film can be arbitrarily selected. On the other hand, the fine
particle catalyst can be formed by utilizing laser ablation or a
solution reaction. In the case of using a solution reaction,
impurities such as carbon are sometimes attached to the periphery
of the fine particle formed. The impurities are generally
evaporated and disappear by heat treatment at a high temperature of
500.degree. C. or more, however, in some cases, the impurities
cannot be completely removed by this heat treatment only. In such a
case, the remaining impurities can be removed by, for example, an
annealing treatment using a reactive gas such as hydrogen. The
annealing can be performed under conditions the same as or close to
the temperature and pressure conditions at the growth. Therefore,
this treatment can be performed in the growth chamber before the
growth of carbon nanotubes is started, and, subsequently, the
growth of carbon nanotubes can be performed in the same growth
chamber.
In the present invention, a hot-filament is used, whereby the
carbon nanotube can be orientation-grown at a relatively low
temperature. According to the method of the present invention, a
temperature of 600.degree. C. or less of the growth surface of the
substrate is sufficient for obtaining carbon nanotubes. The
temperature of the growth surface of the substrate may even be less
than 500.degree. C.
As for the substrate on which carbon nanotubes are grown, for
example, a substrate of a semiconductor represented by silicon can
be used. As described above, the temperature of the substrate
surface can be relatively low and therefore, a substrate material
which cannot be used as the substrate for the growth of carbon
nanotubes in conventional CVD methods, such as a glass substrate,
can also be used.
In producing carbon nanotubes by the present invention, an
apparatus where a growth substrate is contained and a starting
material gas is supplied to orientation-grow carbon nanotubes on
the substrate by CVD and where a hot filament is equipped in the
growth chamber, is used.
EXAMPLES
The present invention is described in greater detail below by
referring to Examples, however, the present invention is not
limited thereto.
Example 1
In this Example, a case where the present invention is applied to
an electronic device having a wiring via is described.
As shown in FIG. 5A, an SiO.sub.2 interlayer insulating film 103
(500 nm) is deposited on a lower wiring Cu layer 101 on a substrate
(not shown) and thereon, a resist pattern (not shown) having
openings, in the regions at which wiring vias are to be formed, is
formed. By using the resist pattern as the mask, a wiring via 105
is formed in the interlayer insulating film 103. Thereafter, a Ti
layer (50 nm) and an Ni catalyst metal layer (10 nm) are deposited
in this order on the entire surface of the substrate by sputtering
or evaporation. Subsequently, a Ti layer 107 (50 nm)/Ni layer 109
(10 nm) laminate film is caused to remain in the wiring via by the
lift-off method using the resist film. In place of the Ni layer, a
layer formed of Fe or Co may be used as the catalyst metal layer or
a layer of an alloy containing at least one of Ni, Fe and Co may
also be used. Also, in place of the thin layer of catalyst metal, a
fine particle may be used. Furthermore, in place of the Ti layer,
an Nb layer, an Si layer or a graphite carbon layer may be
used.
In the wiring via 105, carbon nanotubes 111 are grown by CVD (see,
FIG. 5B). For the growth of carbon nanotubes, for example, thermal
CVD is used. In this case, the substrate is placed in a vacuum
chamber (reaction chamber) and, for example, a mixed gas of
acetylene and hydrogen at flow rates of 80 sccm and 20 sccm,
respectively, is introduced as a reaction gas into the vacuum
chamber and the pressure and substrate temperature are set to 200
Pa and 900.degree. C., respectively. Also, hot-filament CVD, for
performing gas dissociation by a hot filament, may be used. In this
case, for example, a mixed gas of acetylene and hydrogen at flow
rates of 80 sccm and 20 sccm, respectively, is introduced as a
reaction gas into the vacuum chamber and the pressure, substrate
temperature and hot filament temperature are set to 1,000 Pa,
600.degree. C. and 1,800.degree. C., respectively. Alternatively,
DC plasma enhanced hot-filament CVD, combining direct-current (DC)
plasma and a hot filament, may also be used. In this case, for
example, a mixed gas of acetylene and hydrogen, at flow rates of 80
sccm and 20 sccm, respectively, is introduced as a reaction gas
into the vacuum chamber and the pressure, substrate temperature and
hot filament temperature are set to 1,000 Pa, 600.degree. C. and
1,800.degree. C., respectively. In order to vertically orient the
carbon nanotubes, a direct-current (DC) electric field of -400 V
was applied to the substrate with respect to the chamber
(grounded). The application of direct-current (DC) electric field
is advantageous for obtaining perpendicularly oriented carbon
nanotubes with respect to the substrate. The carbon nanotubes 111
are grown by taking the Ni fine particle 109a into the inside at
the root from the Ni layer 109 and ohmically connected to the Ti
layer 107 through TiC generated by the partial carbidization of Ti
in the lower layer 107.
Before the growth of carbon nanotubes, the substrate in the vacuum
chamber may be heat-treated (for example, at 600.degree. C. for 30
minutes) in vacuum or in a hydrogen atmosphere to alloy the Ni/Ti
laminate film. By this heat treatment, an Ni fine particle, a Ti
layer and an NiTi alloy layer are formed in this order from the
film surface. Thereafter, the carbon nanotube 111 is grown by CVD
while incorporating the Ni fine particle in the uppermost part as
the catalyst metal into the inside of the tube side wall and at the
same time, the side wall of the nanotube is joined with the Ti
layer 107a lying beneath.
Then, as shown in FIG. 5C, a Ti layer 113 (50 nm) and a Cu layer
115 (500 nm) are deposited in this order by sputtering or
evaporation. Subsequently, a heat treatment (for example, 500 to
800.degree. C. for 30 minutes) is performed to cause TiC formation
in the upper end of the carbon nanotube 111. As a result, the
carbon nanotube 111 was connected by ohmic contact to the upper and
lower wiring layers 101 and 115.
Example 2
In this Example, a case where the present invention is applied to
an electronic device having a transverse wiring is described.
As shown in FIG. 6A, an SiO.sub.2 insulating film 123 (500 nm) is
deposited on an Si substrate 121 and thereon, a resist pattern (not
shown) having openings in the regions, at which electrodes are to
be formed, is formed. On the entire surface of the substrate, a Ti
layer (50 nm) and an Ni catalyst metal layer (10 nm) are deposited
in this order by sputtering or evaporation. Subsequently, only a Ti
layer 125 (50 nm)/Ni layer 127 (10 nm) laminate film (electrode
pattern) in the openings of the resist pattern is caused to remain
by the lift-off method using the resist film.
Between a pair of opposing electrode patterns, a carbon nanotube
129 is grown by CVD (see, FIG. 6B). The CVD and growth conditions
used may be the same as those described in Example 1. Also,
similarly to Example 1, the Ni/Ti laminate film may be heat-treated
in advance of the growth of carbon nanotube by CVD. In order to
transversely orient the carbon nanotube in parallel to the
substrate surface, a direct-current (DC) electric field of 400 V
was applied between electrodes. The carbon nanotube 129 is grown by
incorporating a part of Ni in the Ni layer 127 as a catalyst
element 127a into the inside and is ohmically connected to the Ti
layer 125 through TiC generated by partial carbidization of Ti in
the Ti layer 125.
Example 3
As shown in FIG. 8A, an Ni thin film 222 for catalyst was formed on
a silicon substrate 220 by evaporation and, thereon, an SiN
insulating film 224 (thickness: 500 nm) having an opening with a
diameter of 2 .mu.m was formed. The thickness of the Ni thin film
222 can be arbitrarily decided, but the thickness was set here to 2
nm. This substrate was introduced into a reaction part (vacuum
chamber) (not shown) and the substrate temperature was set to
500.degree. C. A starting material gas prepared by mixing argon and
acetylene at a ratio of 80:20 was supplied to the reaction part at
a flow rate of 100 ccm and the total gas pressure in the reaction
part was adjusted to 1 kPa by the control in an exhaust system
connected to a vacuum pump. A hot filament (made of rhenium) was
moved above the substrate and with a distance of about 6 mm to the
substrate, a current of about 8 A was passed. By this passing of a
current, the hot filament temperature was elevated to 800.degree.
C. After maintaining this state for 1 minute, the passing of the
current to the hot filament was stopped. The remaining starting
material gas in the reaction part was vacuum-evacuated and the
substrate was taken out from the reaction part. As shown in FIG.
8B, carbon nanotubes 226 having a length of about 2 .mu.m were
formed in the vertical direction from the substrate surface.
Example 4
As shown in FIG. 9A, a film of Fe fine particles 232 having a
diameter of 7 nm was formed by laser ablation in an opening with a
diameter of 2 .mu.m of an SiN insulating film 234 (thickness: 500
nm) formed on a silicon substrate 230. This substrate was
introduced into a reaction part and the substrate temperature was
set to 400.degree. C. A starting material gas, prepared by mixing
argon, acetylene and hydrogen at a ratio of 24:6:70, was supplied
to the reaction part and the total pressure in the reaction part
was adjusted to 1.3 kPa. A current of about 0.7 A was passed to a
hot filament moved above the substrate and the hot filament
temperature was set to about 400.degree. C. After maintaining this
state for 15 minutes, the passing of a current was stopped. The
remaining starting material gas in the reaction part was
vacuum-exhausted and the substrate was taken out from the reaction
part. As shown in FIG. 9B, carbon nanotubes 236 having a length of
about 2 .mu.m and a diameter of about 15 nm were formed in the
vertical direction from the substrate surface.
Example 5
As shown in FIG. 10A, a film of FePt fine particles 242 having a
diameter of about 4 nm was formed on a glass substrate 240 by a
solution reaction. Due to the solution reaction, the periphery of
the FePt fine particle was covered with impurities such as carbon.
Usually, most impurities are evaporated and disappear by a heat
treatment of 500.degree. C. or more, but this is not sufficient for
using the FePt fine particle as the catalyst for the growth of
carbon nanotubes. Therefore, after placing the substrate in a
reaction part, hydrogen was introduced to adjust the pressure to 1
kPa and the substrate was annealed at 500.degree. C., thereby
completely cleaning the FePt fine particles.
Thereafter, a 95:1:4 mixed gas of hydrogen, acetylene and argon was
supplied to the reaction part and the total pressure in the
reaction part was set to 1 kPa. A current of 10 A was passed
through a hot filament moving above the substrate heated to
500.degree. C. and this state was maintained for 10 minutes. As
shown in FIG. 10B, carbon nanotubes 244 having a length of about 2
.mu.m and a diameter of about 8 nm were formed in the vertical
direction from the substrate surface.
As described, the present invention provides a structure where a
carbon nanotube is connected by a good ohmic contact to a metal
material (for example, electrode material or wiring material) which
is the objective of connection. This ohmic connection structure is
formed simultaneously with the growth of carbon nanotube and
therefore, this can dispense with a step of forming an ohmic
connection structure, which has been conventionally performed in
the lower junction part of via of an electronic device after the
growth of nanotubes. Accordingly, in the case of applying the
carbon nanotube as a longitudinal wiring via material, particularly
for ULSI wiring, a good ohmic connection, which has been heretofore
difficult to produce, can be formed between the nanotube and the
lower junction part.
Furthermore, according to the present invention, carbon nanotubes
orientation-grown at a low temperature on a substrate can be
obtained without relying on auxiliary means which has been
heretofore used, such as electric field or plasma.
* * * * *